1. Technical Field
The present invention generally relates to oxygen sensor signal filtering and, more particularly, to a method of selectively forcing an oxygen sensor signal to a high or low signal state.
2. Discussion
Modern automotive vehicles are commonly equipped with oxygen sensors in the exhaust system. The oxygen sensors indicate a lean or rich operating condition by sensing the amount of oxygen in the emissions. Switching type oxygen sensors provide a voltage which is either low or high depending upon the amount of oxygen in the system.
A switching type oxygen sensor emits a low voltage signal under a lean condition and a high voltage signal under a rich condition. Depending upon the signal received from the oxygen sensor, the engine controller can vary the fuel to air ratio within the vehicle engine to vary the emissions output. As such, closed loop or feedback control is established.
The sensitivity of modern oxygen sensors allows detection of lean and rich conditions at an extremely high frequency. For example, modern oxygen sensors can sense the varying conditions within the emissions caused by individual cylinder firing events. Since such switching is not associated with the true chemical condition of the emissions, the type of switching is commonly known in the art as chemical noise.
Chemical noise causes the output of the oxygen sensor to be somewhat unreliable. That is, the oxygen sensor may switch between low and high voltage signal states due to an individual cylinder firing event where over a greater time period the true condition of the emissions may not be accurately reflected in the signal. Such xe2x80x9cfalsexe2x80x9d switching may lead to a variation in the fueling of the engine which would otherwise be unnecessary.
Conventional attempts to reduce false switching include changing the frequency of the oxygen sensor signal and also filtering out voltage spikes. One such attempt averages the input of the oxygen sensor signal. By slowing down the filter rate, the output signal experiences a change in frequency and a decrease in noise level. Unfortunately, such output signals are too slow for most operating systems. As such, the system can not reliably detect sensor signal switching between a low signal state and a high signal state.
Another attempt to reduce false switching involves the detection of the slope of the input signal. When enough of a positive slope is detected, the output signal is forced high. When enough of a negative slope is detected, the output signal is forced low. Unfortunately, the output signal still has noise in it and this technique does not have a significant impact on the frequency of the oxygen sensor signal.
In view of the forgoing, there continues to be a need in the art for a method of filtering an oxygen sensor signal so that reliable switching between the low and high voltage signal states can be readily detected.
The above and other objects are provided by a method of filtering an oxygen sensor signal. The method includes obtaining an oxygen sensor signal from the oxygen sensor on a periodic basis. The oxygen sensor signal is then compared to the average oxygen signal voltage. If the oxygen sensor signal is greater than the average oxygen signal voltage, a high signal counter is incremented. If the high signal counter is greater than a signal count threshold, the oxygen sensor signal is forced to a high signal value. If the oxygen sensor signal is less than the average oxygen signal voltage, a low signal counter is incremented. If the low signal counter is greater than a signal count threshold, the oxygen sensor signal is forced to a low signal value. The high and low signal count thresholds correspond to a preselected period of time indicating a low or high signal trend within the oxygen sensor signal.